1. Field of the Invention
The present invention relates to methods and systems for controlling power to be fed to electrical loads. More particularly, the present invention relates to methods and systems for controlling the output of a generator to make it follow variations in power demands of electrical loads.
2. Description of the Related Art
As generators for vehicles, alternators are commonly used. An alternator has field windings and can regulate a field current flowing therethrough so as to control the alternator output (output power). The alternator output is supplied to vehicle electrical loads, referred to simply as loads hereinafter, and a battery.
For example, the loads include an electrically assisted turbocharger. The electrically assisted turbocharger is operative to boost-harge air in an engine using wasted energy in the exhaust gas thereof, which will contribute to downsizing of the engine. The engine's downsizing will provide reduction in size and weight of the vehicle and improve the fuel efficiency of the engine. Specifically, even if demand for sudden acceleration occurs, the electrically assisted turbocharger can add electric energy (assist power) to an electric motor thereof to electrically assist rotation of a turbocharger's turbine. This allows the rotation of the turbine to match fuel and boost for the desired power output corresponding to the sudden acceleration, making it possible to reduce “turbo lag” in developing power in the engine for sudden acceleration.
As an example of controlling electrically assisted turbochargers, Japanese Unexamined Patent Publication No. H7-11965 discloses a control apparatus. The control apparatus is operative to cause a battery to feed the maximum current to a motor of an electrically assisted turbocharger at the early stages of electric driving of the motor so that the boost pressure reaches a predetermined pressure in order to improve the response of the turbocharger to sudden acceleration of the engine.
In the control apparatus disclosed in the Patent Publication, a cycle of supplying high power from the battery to the motor and, thereafter, of charging the battery by the alternator is needed to be repeated every driver's requirement of power output corresponding to sudden acceleration. This may cause deterioration of the battery to be accelerated.
This problem set forth above appears prominently when supplying the maximum current (assist power) from the battery to the motor of the electrically assisted turbocharger in order to improve the response of the turbocharger to sudden acceleration of the engine.
Even if feeding the maximum power to the electrically assisted turbocharger immediately in response to the start of operation thereof, an inertial delay of the turbocharger and a high power loss at the start of operation of the motor may reduce the efficiency in the use of the power fed to the turbocharger.
Note that, upon decrease in the power supply voltage due to discharge of the battery, the control apparatus executes a compensating operation of controlling the alternator to increase the output thereof, thereby compensating the discharge of the battery and the decrease in the power supply voltage. If feeding the maximum power to the electrically assisted turbocharger immediately in response to the start of operation thereof, the compensating operation cannot keep up with the discharge of the battery. This may make it difficult to prevent the discharge of the battery and the decrease in the power supply voltage.
On the other hand, the alternator output is needed to match the amount of power demands of loads in the long run. A short-term mismatch between the alternator output and power requirements of loads, such as load currents, is tolerated under charge and discharge load of the battery connected in parallel to the loads. It is preferable that the alternator output quickly follows changes in power requirements of the loads in view of reduction of the charge and discharge load of the battery.
As an example of control of the alternator output, alternator output control with maintenance of battery voltage has been commonly used. When a mismatch between the alternator output and the amount of power requirements of loads occurs, the alternator output control with maintenance of battery voltage is configured to cause the battery to charge and/or discharge in order to make up for the mismatch. When the charge and/or discharge of the battery results in that the battery voltage varies, the alternator output control with maintenance of battery voltage is configured to detect the battery voltage variation to regulate the alternator output based on the detected result.
As another example of control of the alternator output, Japanese Unexamined Patent Publication No. S58-192499 discloses alternator output control configured to constantly measure the total amount of load currents of loads as the amount of power requirements of loads, and to supply the amount of a field current, which allows the alternator to generate electric power as much as the measured amount of power requirements. This alternator output control disclosed in the Patent Publication No. S58-192499 is referred to as “current coincident alternator output control” hereinafter.
The current coincident alternator output control has an advantage in which the charge and discharge load is reduced because it is possible to regulate the alternator output such that it follows changes in the amount of power requirements of loads with high response, as compared with the alternator output control with maintenance of battery voltage.
In addition, in order to make up for the practically unavoidable gap between the amount of power requirements of loads and the alternator output, the Patent Publication No. S58-192499 discloses a method of, when the variation in battery voltage exceeds a predetermined range, gradually correcting the amount of filed currents to cancel the battery voltage variation to regulate the alternator output.
As a further example of control of the alternator output, U.S. Patent Publication No. 4,636,706 corresponding to Japanese Unexamined Patent Publication No. S62-64299 discloses alternator output control configured to positively use the mismatch toleration effect between the alternator output and the amount of power requirements of loads by the battery.
Specifically, this control is configured to forcibly reduce sudden changes of a field current in the alternator so as to prevent the load torque of the alternator from steeply changing. The control disclosed in the Patent Publication No. S62-64299 is referred to as “field-current reduction alternator output control”. The field-current reduction alternator output control may however cause an unacceptable drop in the battery voltage due to delay of the changes in the field current.
Against the background of the control disclosed in the Patent Publication No. S62-64299, Japanese Unexamined Patent Publication No. H05-260679 discloses a method of prohibiting the field-current reduction alternator output control when booting a predetermined load. In the control disclosed in the Patent Publication No. H05-260679, however, when the load currents abruptly change during prohibition period of the field-current reduction alternator output control, the alternator output significantly changes for canceling the abrupt changes of the load currents. This may cause voltage hunting to easily appear.
In the current coincident alternator output control disclosed in the Patent Publication No. S58-192499, in order to reduce changes in the battery voltage due to an accumulation of the gap between the measured amount of power requirements and the alternator output corrected to correspond thereto, the field current to be supplied to the alternator is controlled when the changes in the battery voltage exceed a predetermined range.
While the field current to be supplied to the alternator is controlled for canceling the changes in the battery voltage, however, the response of the regulation of the alternator output to the changes in the amount of power requirements of loads may deteriorate. This may reduce the advantage obtained by using the current coincident alternator output control.